Before the sun rose this morning, the ROPOS team was prepping the remotely operated vehicle for our first research dive.
The main target was a place nearly devoid of oxygen where, surprisingly enough, some animals are still able to thrive. What they are able to do, a species of sole in particular, inspired a strange fishing trip of sorts, and may give some clues about what to expect as low-oxygen areas in the region seem to be expanding.
We’re in spectacular Saanich Inlet, northwest of Victoria. Every summer for thousands of years, natural processes have caused oxygen levels in the deeper waters of this fjord to approach zero. But as the water gets shallower, oxygen levels rise. The main focus of this expedition is to study low-oxygen waters offshore and the ways this water’s reach may be expanding. But Saanich offers a more controlled environment for learning about low oxygen’s effects on animals by comparing what’s found at the different depths.
“The oxygen is so low it boggles the mind how that animal can survive down there.”
Sole Survival
The dive began in the central inlet, about 180 meters down (watch a highlights video below). The oxygen reading from the ROV’s sensor was diving as fast as the vehicle. On the bottom, it was effectively zero and there was no obvious life in sight. But as the oxygen crept up just a tad—to a couple of tenths of a milliliter per liter, which is hundreds of times lower than typical waters—a small flatfish called the slender sole became fairly abundant, showing up repeatedly on the video feed as the ROV made its way along the bottom. “The oxygen is so low it boggles the mind how that animal can survive down there,” says Jackson Chu.
Jackson is hoping to learn more about the sole. One goal for the dive was to collect samples using a net and ROPOS’s mechanical arm. The pilot was able to get a couple of fish into a net, but all escaped. The prized catch wouldn’t come until much later in the day.
The next dive was to a nearby section of the inlet that looked more like what most people think of as fjords—cliffy, forested sides making for beautiful scenes above the water. The sheer rocky cliffs that we’d see below the water were no less striking. Here the main goal was to start at the bottom of two different cliffs and work up from the no-oxygen, or hypoxic lower layers, to the surface.
This allowed the researchers to survey what animals are found in the most inhospitable lower zone to compare against what’s found higher up as oxygen levels rise. Besides more slender sole there was also a small hypoxia-tolerant anemone that may well prove to be a new species, which the team was able to collect for the first time. Other inhabitants higher up included tubeworms, nudibranchs, and other fish species.
Based on records going back 50 years, it looks like the size of the low-oxygen layer is expanding for reasons that aren’t yet clear. So, the animals that can handle it may have an increasing advantage of the majority, which can’t.
It’s not clear how the animals can tolerate the low oxygen, but there was an interesting illustration at one spot. The oxygen was at the moderate level, and a slender sole was barely having to breath, while another sole species was rapidly pulling water through its gills to get enough oxygen. As with other low-oxygen tolerant animals, the slender sole may have some physiological adaptation that allows it to more efficiently pull oxygen from the water.
Gaining an Edge
The soles differing capabilities may offer a hint of things to come. The slender sole is a non-commercial species, mainly because it doesn’t grow large enough. The other sole, which was breathing harder, is a commercial species. So, it’s possible that if low-oxygen waters are expanding, that the slender sole’s advantage may allow it to outcompete other species.
Jackson will also have a chance to study the slender sole’s adaptations more closely. On the day’s third and final dive, which ran through the night, the team took the opportunity to connect a new piece of equipment to the hard wiring of the VENUS observatory. But while redoing the first dive’s transect to explore whether the slender sole are distributed differently during the day and the night, the ROPOS team also got another shot at a slender sole and this time managed to keep one.
The fish, named Vinnie Jr. in honor of the pilot who captured it, was sent quickly to shore via one of the ship’s small boats. Hopefully, Jackson’s colleagues in the lab will be able to keep Vinnie Jr. alive for some experiments once he’s back that will look at how the species handles low oxygen. If those exploratory experiments go well, Jackson hopes to use a trawl to collect more fish that he can study more extensively.
The dives complete, we’re on our way out for a short dive in the Strait of Georgia before we make our way offshore.
